Acetalation of pentaerithritol catalyzed by an Al-pillared saponite

Article abstract of DOI:10.1007/s10562-011-0572-8

Acetalation of pentaerithritol has been carried out in the presence of an Al-pillared saponite. Several carbonyl compounds were used, including aldehydes and ketones. Yields between 72 and 93% were obtained in all cases, except in the case of veratraldehyde where the carbonyl group was blocked. The high conversion could be explained by diffusion effects of intermediates and electronic effects between the reactants in the transition state. The catalyst was found to be active after three cycles.

Full text of DOI:10.1007/s10562-011-0572-8

Catal Lett (2011) 141:1118–1122
DOI 10.1007/s10562-011-0572-8
Acetalation of Pentaerithritol Catalyzed by an Al-Pillared
Saponite
•
•
•
V. Kannan K. Sreekumar A. Gil
M. A. Vicente
Received: 23 December 2010 / Accepted: 22 February 2011 / Published online: 8 March 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Acetalation of pentaerithritol has been carried
out in the presence of an Al-pillared saponite. Several
carbonyl compounds were used, including aldehydes and
ketones. Yields between 72 and 93% were obtained in all
cases, except in the case of veratraldehyde where the car-
bonyl group was blocked. The high conversion could be
explained by diffusion effects of intermediates and elec-
tronic effects between the reactants in the transition state.
The catalyst was found to be active after three cycles.
pentaerythritol tetraacrylate or the explosive pentaerythri-
tol tetranitrate (PETN). Derivatives of pentaerythritol are
components of alkyd resins, varnishes, PVC stabilizers, tall
oil esters, and olefin antioxidants. Halogen-free pentae-
rythritol esters are environmentally friendly alternative to
conventional electrical transformer fluids, being both
readily biodegradable and non-hazardous in water. They
advantageously replace polychlorobiphenyl (PCB), and
even silicone-based or fluorinated hydrocarbons, as their
low volatility and high flash point give them an excellent
resistance to ignition in case of major electrical failure and
transformer rupture [1].
Keywords Pentaerithritol Á Acetalation Á Alumina
pillared saponite
Pentaerithritol acetals are applied as plasticizers and
vulcanizers, as physiologically active substances and as
potential protective groups for aldehydes and ketones [2].
1,2-diacetals are efficient protecting groups for vicinal 1,2-
diol units in carbohydrates [3, 4]. Protection of the car-
bonyl group of aldehydes and ketones can be accomplished
by alcohols [5], diols [6] or trioxanes [7]. Acetonide for-
mation is the most commonly used protection for 1,2(cis)-
and 1,3-diols, which have extensively been used in car-
bohydrate chemistry to selectively mask the hydroxyls of
different sugars. These reactions are generally effected
either with a free carbonyl, aldehyde or ketone [8], or with
masked carbonyls (e.g., acetals [9], ketals [10], or enole-
thers [11]) in the presence of a variety of catalysts such as
mineral acids [12], formic acid [13], CuSO4 [14], ZnCl2
[15], p-toluenesulfonic acid [16], camphorsulfonic acid
[17], iodine [18], etc. Besides the interest of acetals as
protecting groups, many of them have direct application as
fragrances [19, 20], in cosmetics and beverage additives
[21, 22].
1 Introduction
Pentaerythritol is an alcohol with formula C(CH2OH)4 (see
Scheme 1). It is a white, crystalline polyol with the neo-
pentane backbone, a versatile building block for the
preparation of many polyfunctional compounds such as
V. Kannan Á K. Sreekumar
Department of Applied Chemistry, Cochin University of Science
and Technology, Cochin 22, Kerala, India
e-mail: kannanpvl@gmail.com
K. Sreekumar
e-mail: ksk@cusat.ac.in
A. Gil
Department of Applied Chemistry, Public University of Navarra,
31006 Pamplona, Spain
e-mail: andoni@unavarra.es
M. A. Vicente (&)
Department of Inorganic Chemistry, University of Salamanca,
37008 Salamanca, Spain
e-mail: mavicente@usal.es
The most general method for the synthesis of acetals is
the reaction between carbonyl compounds and an alcohol
or an orthoester in the presence of acid catalysts. The
123

Acetalation of Pentaerithritol
1119
CHO
Scheme 1 Reaction of a
carbonyl, benzaldehyde is taken
as example, and pentaerithritol
to form the corresponding
diacetal
HO
HO
OH
OH
O
O
O
O
2
commonly used acid catalysts include corrosive protic
acids such as HCl, H2SO4 and Lewis acids such as ZnCl2
and FeCl3 [23, 24]. A series of diphosphine complexes
have also been employed for the synthesis of acetals [25,
26]. However, acetalation by afore mentioned methods
require tedious work-up procedures, and the chemicals
required for the synthesis of complexes are expensive. In
this context, the use of heterogeneous acid catalysts for the
reaction is attractive and it may allow carrying out it
without the generation of wastes.
The slurry was stirred for 24 h at room temperature and
centrifuged and washed by dialysis with distilled water,
dried in air at 70 °C for 16 h and calcined at 500 °C for
2 h, at a heating rate of 5 °C/min, in order to obtain the
alumina-pillared saponite. The sample thus synthesized
was designated as Al-PILC.
2.2 Instrumental
X-ray diffraction (XRD) patterns of non-oriented powder
samples, between 2° and 65° of 2h with a scanning velocity
of 2°/min, were obtained on a Siemens D-500 diffrac-
tometer with Ni-filtered Cu-Ka radiation, at 40 kV and
30 mA.
Pillared interlayered clays (PILCs) are two dimensional
zeolite-like materials prepared by exchanging compensat-
ing cations between the clay layers with large inorganic
oligomeric metal hydroxycations, which are formed by
hydrolysis of metal oxides and salts. When these materials
are compared to other porous solids, pillared clays offer
several advantages; they are obtained from cheap sources
and are synthesized under moderate conditions with much
simple procedures compared to zeolite synthesis [27–29].
Al-pillared clays are efficient solid acid catalysts for
liquid phase processes due to comparatively strong acidity
[27–33], which makes clay-based catalysts very attractive
for acid-catalyzed reactions. Thus, the use of an Al-pillared
saponite as catalyst in the acetalation of pentaerithritol is
explored in the present work.
Elemental analysis of the solids was carried out by
inductively coupled plasma optical emission spectroscopy
(ICPS OES) at Activation Laboratories Ltd., Ancaster,
Ontario, Canada.
Nitrogen (Air Liquide, 99.999%) adsorption experi-
ments were performed at -196 °C on a static volumetric
apparatus (Micromeritics ASAP 2010 adsorption analyzer).
The samples, 0.2 g, were previously degassed at room
temperature for 6 h and then for 2 h at 110 °C at a pressure
lower than 0.133 Pa. The specific total surface area (SBET
)
was calculated using the BET equation from adsorption
data in the relative pressure range between 0.05 and 0.20,
and the micropore volume was obtained by means of the
t-method [37].
2 Experimental
2.1 Materials
2.3 Catalytic Performance
The natural clay mineral used in this work was a saponite
from Caban˜as (Toledo, Spain), supplied by TOLSA
(Madrid, Spain). The raw material was previously purified
by dispersion–decantation, obtaining the 2 lm fraction
[34].
Acetalation of pentaerythirtol was carried out in a 50 cm3
two necked glass batch reactor connected to a Dean-Stark
apparatus. The reaction was carried out in nitrogen atmo-
sphere. Solutions of pentaerithritol (1 mmol), aldehyde or
ketone (2 mmol) in 20 cm3 of toluene and 0.25 g of cata-
lyst are stirred and refluxed. The progress of the reaction
was monitored by thin layer chromatography on silica gel
coated plates (Merck 60 F-254) using hexane:ethyl acetate
mixture (10:2 v/v). After completion of the reaction the
products were obtained by evaporating the solvent in a
rotary flash evaporator and subsequent recrystallization
from ethanol gave the corresponding products.
The parent clay was intercalated with aluminium
polycations, [Al13O4(OH)24(H2O)12]712?, in short Al13
7?
,
according to a procedure described previously [35, 36]. An
aluminium polycation solution was prepared by slow
addition of a solution of NaOH (Panreac, p.a.) to a solution
of AlCl3Á6H2O (Panreac, p.a.), under vigorous stirring,
with a mole ratio OH-/Al3? equal to 2.2. The resulting
solution was left to age for 24 h at room temperature under
constant agitation. The solution was added to a previously
prepared clay suspension at a ratio Al/clay of 5 mmol/g.
Pentaerithritol was obtained from Loba Chemie Pvt.
Ltd., Mumbai (India), while all the aldehydes and ketones
were obtained from Merck Ltd. India, and were used as
123

1120
V. Kannan et al.
received. The solvents were of analytical grade or were
purified by standard procedures prior to use.
Table 1 Chemical composition of natural and pillared saponite
Sample SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2
The products were characterized by FT-IR and NMR
spectroscopies. FT-IR spectra were recorded by the KBr
pellet method on a JASCO FT-IR spectrophotometer in the
Saponite 60.09 5.77 1.65
Al-PILC 56.46 14.56 1.51
0.03 30.70 0.71 0.27 0.52 0.25
0.03 26.64 0.11 0.04 0.44 0.22
1
range of 400–4000 cm-1. H NMR spectra were recorded
Results are given on dry-basis, that is, the content of the metallic oxides has
been normalized to sum 100%
on a Bruker 400 MHz instrument with tetramethylsilane
(TMS) as internal standard in CDCl3.
3 Results and Discussion
3.1 Characterization of the Catalyst
Pillaring of the original saponite with aluminium polyca-
tions was carried out following a standard method. In fact,
Al-pillared Yunclillos–Caban˜as saponite has already been
used in some previous works [34]; in which more infor-
mation about this clay can be found. The essential char-
acterization data on the catalyst synthesized for the present
work are reported here.
The process of intercalation-pillaring was successful,
˚
giving rise to pillared clay with a basal spacing of 18.0 A,
Fig. 2 FTIR spectra of pillared saponite (P) and this sample after
adsorption of pyridine (PA)
as observed in Fig. 1. This indicates that the catalyst used
in this work is a well-ordered layered solid, with its
interlayer space considerably increased by the pillaring
treatment. All the characteristic peaks of the original clay
are maintained in the pillared one, showing that the
structure of the layers was not altered; simply they were
separated on the c-direction. Chemical analyses (see
only shows a proportional decrease due to the fixation of
aluminium. The specific BET surface area of the pillared
solid is 274 m2/g, from which 218 m2/g (80%) correspond
to micropore region, showing a correct development of the
porous structure of the solid during the pillaring processes.
Characterization by thermogravimetry, differential thermal
analysis, FT-infrared spectroscopy, etc. [34], -not shown
for simplicity-, shows that all the properties of the catalyst
are typical for pillared saponite.
Table 1) show the fixation of about 8.8 wt.% Al2O3 during
7?
the pillaring, Al13 polycations have substituted Ca2?
,
Na?, and in lower extent K?, the exchangeable cations of
the original clay. The percentage of the rest of the elements
The investigation of surface acidity of Al-PILCs by IR
spectroscopy using pyridine as probe molecule has been
successfully used in various investigations [33, 38]. The
spectra of pillared saponite, before and after pyridine
adsorption, are shown in Fig. 2.
1000 cps
The spectra show adsorption bands at 1449, 1492 and
1600 cm-1, which are assigned to pyridine molecule
¨
interacting with Lewis and Bronsted acid sites of the cat-
Al-PILC saponite
¨
alyst. It has been shown in earlier reports that Bronsted
acid sites in the vicinity of Lewis acidic metal center
(active sites) act as co-catalyst [39, 40]. The bands at
1449 cm-1 and 1600 cm-1 are attributed to pyridine bound
to Lewis acid sites (L) [41, 42]. The band at 1492 cm-1
may be attributed to pyridine associated with both Lewis
and Bronsted acid sites (L ? B). It is observed that the
peak corresponding to Lewis acid sites (1449 and
1600 cm-1) are more intense than the peak corresponding
Saponite
10
20
30
40
50
60
2θ (degrees)
Fig. 1 X-ray diffractograms of the parent purified saponite and of the
Al-pillared catalyst
to Bronsted and Lewis acid sites (1492 cm-1).
¨
123

Acetalation of Pentaerithritol
3.2 Catalytic Performance
1121
OS
H
OH
HO
OH
OH
OS
O
OH
O
The reaction studied is shown in Scheme 1, in which
benzaldehyde is taken as the example of carbonyl com-
pound. The substrates considered in each case are those
listed in Table 2, in which the yield obtained for each
reaction is also given.
HO
H
OS
S
H
O
O
OH
O
O
OH
O
Before discussing the results obtained, it may be said
that acetal formation is believed to proceed through
hemiacetal intermediates (see Scheme 2) [43]. The car-
bonyl compound is first attacked by the Lewis acid sites of
the pillared clay catalysts which provide enough space to
diffuse in the interlayer and successive formation of
intermediates which combines with hydroxyl group to form
the hemiacetal; deprotonation and subsequent elimination
of metal hydroxides gives the corresponding acetal. The
rate determining step of acetalation is the formation of
cation from the protonated hemiacetal. The bulkiness of
hemiacetals might prevent the attack of the second
hydroxyl group thereby changing the rate determining step.
Other factor, the electron withdrawing power of phenyl
group in these compounds, reduces the easy release of the
pair of electrons on the carbonyl carbon during the
reaction.
HO
HO
O
H
S
-S+
O
O
O
O
OH
O
+ H2O
O
HO
Scheme 2 Proposed mechanism for the reaction catalyzed by the
acidity of the pillared saponite (S active positions in saponite catalyst)
electronic factors. The bigger pore size of pillared clays
promotes the rate of diffusion of the bulky products
through the channels resulting in higher yields as compared
to zeolites. The yield of the products correlates well with
the expected electronic effects between the reactants and
also with the polarity of C=O carbonylic group.
From the results presented in Table 2 it is evident that,
in general, aldehydes undergo nucleophilic addition more
readily than ketones. The formation of acetals using pil-
lared clays as catalysts can be explained by two factors,
diffusional properties of the substrates and electronic fac-
tors. As pointed out by Corma et al. [44], diffusional
properties of ketones are the deciding factor rather than
Conversion of veratraldehyde was studied in order to
analyse the influence of electronic and steric effects in the
yield of the reaction. In this case, the oxonium ion formed
in the third step is stabilized by the presence of methoxy
groups in the 3,4-positions. This is further supported by the
fact that the carbonyl group may be completely blocked
(steric effect) and no reaction occurs, which clearly illus-
trates the importance of the accessibility of the reactive
groups of the reactants. This fact is supported by the results
of Thomas et al. [45], who pointed out, by molecular
docking simulation studies of cyclohexanone, acetophe-
none and benzophenone, that as the size of the ketone
increases the acetalation efficiency decreases.
Table 2 Preparation of diacetals from various carbonyl compounds
catalyzed by pillared saponite
Substrate
Time (h) Yield (%) Melting point (°C)
Found Reported
For comparative purposes, the reaction was also carried
out using natural saponite as catalysts, and two selected
representative substrates, namely benzaldehyde and
cyclohexanone. The yields obtained were 82 and 79%,
respectively, clearly lower than the results obtained when
using the pillared solid (93 and 90%), showing that the
pillaring process clearly improves the catalytic activity of
the clay. Besides, a blank run (without catalyst) was carried
out, and no acetal formation was observed, confirming the
role of the clay as catalyst of the reaction.
Benzaldehyde
2
4
5
3
3
2
3
4
8
3
3
3
8
5
93
74
72
78
68
82
84
82
–
156
183
163
156
–
4-methylbenzaldehyde
2-nitrobenzaldehyde
3-nitrobenzaldehyde
4-methoxy benzaldehyde
4-hydroxy benzaldehyde
4-chlorobenzaldehyde
4-bromobenzaldehyde
Veratraldehyde
163
71–72 72–73
155
170
141
213
–
155
171
141
–
–
Between the aldehydes considered, it can be observed
that the conversion increased following the trend ortho-
\ meta- \ para-substituted compounds, and between the
para-substituted compounds, the influence of the nature of
the substituent in the conversion varies as methoxy \
hydroxy = bromo \ chloro.
Cyclopentanone
86
90
82
24
63
137
116
182
162
149
–
Cyclohexanone
116
183
164
147
4-terbutyl cyclohexanone
Benzophenone
Acetophenone
123

1122
V. Kannan et al.
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4 Conclusions
In the present study a mild, efficient and environmentally
benign protocol for the preparation of cyclic acetals using a
heterogeneous catalyst has been presented. Although there
are few reports on acetalation using clays, alumina pillared
saponite shows better yields than the reported catalysts.
Cyclohexanone reacts smoothly with pentaerithritol at
refluxing temperature with pillared clay catalysts. Bulkier
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Acknowledgments The authors thank SAIF Cochin and NMR
Research Centre IISc Bangalore for spectral analysis. One of the
authors (VK) is grateful to UGC New Delhi for a teacher fellowship.
AG and MAV thank the financial support from Spanish Ministry of
Science and Innovation, MICINN, and European Regional Develop-
ment Fund, ERDF (references MAT2007-66439-C02 and MAT2010-
21177-C02).
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